Well, to quibble slightly, if you're running under mod_perl you're not running a CGI--you're running either under Apache::Registry or as a custom handler, I would assume. But anyway...

How do I do debugging in mod_perl? Loads of warn statements, usually. :)

Seriously: while I do actually like using the perl debugger from the command line, I've never played around much with using it from mod_perl. I just rely on writing things to the error log, then taking a look at it; using a per-request trace variable that I can write to and view at the end of a request; etc.

And it really depends *what* you're trying to debug. If you're trying to debug mod_perl itself--if something's going wrong internally that you can't figure out--try turning on mod_perl trace. You can enable the trace for various pieces of mod_perl operation (configuration setup, request handling, etc.), so it can be useful.

If you're debugging stuff about the environment, the request, etc. take a look at Apache::DebugInfo. Or you might want to look at the Debugging mod_perl section of the guide.

But odds are you're debugging something that you wrote. In such cases, as I said, I just tend to use the old standby: print and warn statements. :)